In order to devise a practical, quick assay for serotyping human rotavirus isolates, repeated attempts at isolating monoclonal antibodies directed at the major neutralization protein of human rotaviruses have been made. In the past, monoclonal antibodies directed at the outer capsid proteins VP3, the hemagglutinin, and VP7, the major neutralization protein, of RRV were isolated. Screening by hemagglutination-inhibition assay was of key importance in identifying monoclones directed at the outer capsid proteins of RRV. In order to isolate monoclonal antibodies directed at the major neutralization proteins of human serotypes 1, 2, and 4, Balb/C mice were immunized with human rotavirus x RRV reassortants which had human serotype specificity but contained the 4th gene (the hemagglutinin VP3) and also the remaining genes from RRV. The mice were immunized approximately 3-4 times over a period of 2-4 months with partially purified virus. The fusion ratio was 10 spleen cells per NS-1 mycloma cell. Screening of several fusions by hemagglutination-inhibition identified some monoclones which inhibited hemagglutination of the reassortant rotavirus but all of these monoclones were directed at the RRV 4th gene product, VP3; none were directed at the human rotavirus VP7 protein. A screening test involving neutralization of virus in 96 well tissue culture plates have been developed and will be used to screen for neutralizing antibodies to the different serotypes. In addition, attempts are being made to develop an ELISA test which would enable serotyping of serotype 1 and 3 viruses by using monoclonal antibodies directed at the major neutralization protein of Wa and at the major neutralization protein of RRV (which had been isolated in the past).